Since a polypropylene film has excellent appearance, heat resistance, rigidity, etc., the film is widely used in a packaging field such as food packaging, fiber packaging, etc.
Recently, high-speed film formation by a large-sized film forming machine has been practiced to improve the productivity and also various thicknesses of the films have been employed to widen the purposes of uses. In this case, as a result thereof, a cooling condition at film formation changes, which gives large influences on the transparency of the films formed, and a problem occurs that the transparency of the film, which is good at low-speed film formation or in the thin films having a thickness of from about 20 to 30 .mu.m, is greatly reduced at high-speed film formation or in the films having a thickness of at least 50 .mu.m.
To overcome the problem, an attempt to decrease the surface temperature of a cooling roll by decreasing the temperature of cooling water, an attempt to directly cool a molten resin extruded with water without using a cooling roll, etc., have been made and an effect can be obtained to a certain extent. However, the influences of the temperature of cooling water, the deviation of the flow rate, etc., cannot be avoided and a sufficient effect has not yet been obtained.
Thus, development of a polypropylene composition and a polypropylene film having a good transparency and showing small film-forming condition dependency and also small thickness dependency of the transparency has been desired.
A reforming method of thermally decomposing or peroxide-decomposing a polypropylene resin, a method of compounding a polypropylene resin with a sorbitol nucleating agent, etc., are known as a method of improving the transparency of a polypropylene film.
In the reforming method of decomposing a polypropylene resin, the improvement effect may be obtained at low-speed film formation when the surface temperature of a cooling roll is low, but no effect is obtained at high-speed film formation when the surface temperature of a cooling roll is high. Further, in the case of high-speed film formation of thick films, the transparency is greatly reduced.
Also, in the method of compounding a sorbitol nucleating agent, an effect of improving the transparency may be obtained, but since use of the sorbitol nucleating agent causes an unpleasant odor of the film obtained, there is a restriction on the use thereof.